In recent years, treatments using cell sheets are becoming more widespread. In particular, treatments of transplanting cell sheets made of the patient's own cells have been being established. In such treatments, stem cells are extracted from cells taken from a patient, and myocardial or corneal epithelial/endothelial cells are cultured to create a cell sheet.
The cell sheet needs to be created in a state suitable for transplantation. For example, it is required for the cell sheet for the cornea that cells are properly stratified without a loss and the like. The inspection for this is performed after the culture. Since the cornea is transparent, it cannot be observed with an ordinary microscope. Therefore, phase contrast imaging is performed by a phase-contrast microscope to detect the presence or absence of an abnormality in the cell sheet.
Although inspection using a phase-contrast microscope is simple and the existence of the optical path length differences and the distribution thereof can be observed as irregularities of the pattern due to the phase contrast, it cannot be figured out the degrees of the differences actually exists.
Besides, in the fields of high-density optical memory and optical communication/information processing, for the purpose of understanding the refractive index modulation amount and the refractive index difference, there has been a growing interest in refractive index modulation devices, such as volume phase holographic (VPH) grating, and optical waveguide devices using a refractive index difference. As an example, Patent Document 1 discloses a known phase difference measurement technique. Nowadays, the use of this technique offers a possibility to identify cancer cells and normal cells by quantified indicators.
Meanwhile, as a method for visualizing the internal structure of a biological tissue, optical coherence tomography (OCT) technique is used (e.g., see Patent Documents 2 and 3). There has also been reported a high-resolution optical coherence tomography microscope (OCM) that is achieved by increasing the resolution of this technique to allow the observation of the interior of isolated cells. Such OCT techniques can be used as a method of figuring out the stratification states of the cell sheet. In other words, by using the OCT technology, the cross-sectional shape of layered cell aggregates can be observed with a resolution at a cellular level. Thus, for example, the stratification of the corneal epithelial sheet (basal cell layer/wing cell layer/squamous cell layer) can be checked.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 2008-304456
[Patent Document 2] Japanese Unexamined Patent Application Publication No. 2003-543
[Patent Document 3] Japanese Unexamined Patent Application Publication No. 2007-101365